Traction on a band of flexible material through feed rollers is used in many manufacturing processes wherein the same operation has to be carried out repeatedly or continuously along a band length, or in processes wherein different operations must be carried out sequentially on the same part of a band. In the first case, the band has to pass between the two turning feed rollers that turn in opposite directions and at the same speed in such a way that they pull the band making it move forward continuously by friction and pass, tautly, through the station wherein the operation is carried out, generally located upstream with respect to the rollers. In the second case, the operations take place in various fixed stations, arranged one after the other in the direction of band movement and which act on the aforesaid at the same time as it is forced to move.
In the cases wherein there are two superimposed bands which are pulled, and especially in the cases wherein the bands are stored on reels which unwind when pulled from a free end, due to the differences in forces that the rollers have to tolerate when pulling each one of the bands, and depending on the length of the band still stored on the reel, one of the bands is pulled a greater distance than the other.
Normally these bands have informational or advertising messages printed on them that occupy a certain length on the band and are repeated various times on the reel. The idea is that they are cut during the production process so that each message is finally presented individually, i.e. a single section. Nevertheless, due to the flexibility in plastic sheet production by varying said conditions (raw material, temperature, humidity, stress, etc. . .), small differences in the length of the printed message between one band and another occur. This can and does happen even on the same reel whereby the length of the section of printed message in the initial meters of the reel is not the same as in the central or final meters.
This difference in movement or including dimensions of the printed message on the bands can accumulate as the band moves, which means that along a large length of bands it can cause defects in the finish of the end product, especially in the cases of identical operations that are carried out on the two bands and in those that the bands are paired together after carrying out the aforesaid operations.
When producing mesh bags, heat-sealable material sheets, placed facing each other on each side of a tubular mesh are used to fuse the ends of the bag and thereby form the bottom and seal thereof after being filled. To produce these types of bags, the tubular mesh and the attached sheets have to pass through two feed rollers to carry out the soldering and cutting operations, on the set formed by the mesh and the sheets in order to form the bags.
But the sheets also serve to print identification data on the bag about its content or for advertising. Therefore in the event that there is matching or corresponding information on the sheets adjacent to the two faces of the tubular mesh, there is the possibility that this information does not correspond, once the bag has been finished due to the fact that one sheet has been pulled by the rollers to a different length than the other.
If the two sheets are not lined up it could give rise to other serious consequences that could cause bag production to stop, for example when the sheets have different colours printed on them for the base or for better closing, or if the sheets have markers incorporated designed to be detected by optical readers or any device on the machine.
In any case, due to the fact that progressively and as the rollers pull the tubular mesh to produce one bag after another, the differences in movement between the sheets increase, until such a point that it is necessary to stop the machine to manually line up the two sheets designed to form the two faces of the same bag to avoid the problems stated above.